The present invention relates to a semiconductor thin film applicable to thin film transistors (TFTs) used for liquid crystal displays, memories, and other electronic devices and a fabrication method thereof; an apparatus for fabricating a single crystal semiconductor thin film; and a method of fabricating a single crystal thin film, a single crystal thin film substrate, and a semiconductor device using the substrate.
As semiconductor thin films such as silicon thin films formed on insulating substrates, there have been known an SOI (Silicon On Insulator) structure, and amorphous silicon thin films or polycrystalline silicon thin films formed on glass substrates which have been practically used for liquid crystal displays.
The SOI structure is often formed by way of various steps including a step of sticking single crystal silicon wafers to each other and a step of polishing them, and since the SOI structure basically uses a single crystal silicon wafer, a substantially perfect single crystal portion of the SOI structure can be typically used for a channel portion of an active device of a thin film transistor (TFT). Accordingly, the device thus fabricated can exhibit good electronic characteristics, for example, a high mobility. The method of fabricating the SOI structure, however, requires various steps, for example, a step of sticking single crystal silicon wafers to each other and a step of polishing them, thereby causing disadvantages that the number of steps is increased to prolong the production time, and also the production cost is raised.
On the contrary, there has been known a method of forming a crystallized silicon thin film by depositing a source gas obtained by mixing hydrogen and SiF4 to silane gas on a substrate in accordance with a low-pressure CVD process or a plasma CVD process, and a method of forming a crystallized silicon thin film by forming an amorphous silicon thin film as a precursor on a substrate and crystallizing the amorphous silicon thin film. In the former deposition method in which crystallization of silicon proceeds along with deposition of the silicon thin film, since the substrate temperature is required to be kept at a relatively high temperature, more specifically, 600° C. or more, the substrate must be made from an expensive material withstanding a high temperature such as quartz. In this method, the use of an inexpensive glass substrate may give rise to a problem that the substrate may be deformed or distorted because of its poor heat resistance. With respect to the latter method, as a process of crystallizing an amorphous silicon thin film formed on a substrate, there has been known a solid-phase growth process of annealing the substrate, on which the amorphous silicon thin film has been formed, for a long time (for example, 20 hr). Such an annealing process, however, has a problem that since it takes a long time, the practical utility is poor and also the production cost is raised. To solve these problems, there has been actively studied and developed a method of crystallizing a non-single crystal thin film by irradiation of laser beams emitted from an excimer laser.
This laser irradiation method involves forming an amorphous silicon thin film or a polycrystalline silicon thin film on a substrate, and heating the thin film by irradiation of laser beams emitted from an excimer laser, thereby crystallizing the thin film. For example, in the case of using an XeCl excimer laser, since an emission wavelength is 308 nm and an absorption coefficient is about 106 cm−1, the laser energy is absorbed in a region having a depth of about 10 nm from the surface of an amorphous silicon thin film, with a result that the substrate temperature is little raised, and only a portion near the surface of the amorphous silicon thin film is crystallized.
The technique of melting a non-single crystal thin film by irradiation of laser beams emitted from an excimer laser and recrystallizing the melted thin film can grow polycrystalline silicon grains in an amorphous silicon thin film or a polycrystalline silicon thin film; however, it is very difficult to stably control a crystal quality of the thus formed thin film on the basis of the number of shots of laser beams, thereby tending to cause a variation in threshold voltage of a thin film transistor as a final product.
By the way, in the case of using a PECVD (Plasma-Enhanced CVD) system for forming an amorphous semiconductor thin film on a substrate, the film contains hydrogen in an amount of about 2 to 20 atomic %. In this case, the substrate on which the thin film has been-formed is put in an electric furnace and is subjected to a degassing treatment, for example, at 420° C. for about 2 hr. The hydrogen concentration in the film is reduced to less than 2 atomic % by the degassing treatment.
Such a degassing (annealing) treatment in an electric furnace for removing hydrogen contained in the film has a problem that since the substrate must be annealed, for example, at 420° C. for about 2 hr, the productivity is degraded, and further, the substrate may be deformed due to heat caused by the degassing treatment and a contaminant from glass may be diffused in the thin film.